The present invention relates to a valve timing control system for controlling open-close timing of an intake valve and an exhaust valve of an internal combustion engine, in accordance with engine operating condition.
Moreover, the present invention relates to a method of producing the above mentioned valve timing control system.
Japanese Patent Unexamined Publication No. H9(1997)-324611 discloses a valve timing control system for variably controlling open-close timing of an intake valve and an exhaust valve by rotatably operating an angle at which a timing sprocket (which rotates synchronously with a crank shaft of an engine) is mounted relative to a cam shaft (which has an external periphery formed with a drive cam).
The valve timing control system 14 (referred to as xe2x80x9cVVT mechanism 14xe2x80x9d in Abstract) according to Japanese Patent Unexamined Publication No. H9(1997)-324611 has the following constitution: A cam shaft 13 has an end portion which is integrally mounting a vane member 37 (referred to as xe2x80x9cimpeller 37xe2x80x9d in Abstract). A tubular housing has an external periphery which is integrally formed with a timing sprocket 25 (referred to as xe2x80x9ccam sprocket 25xe2x80x9d in Abstract). A plurality of bulkhead portions 42 are disposed in the tubular housing. Vane member 37 has a vane portion 39 (referred to as xe2x80x9cblade 39xe2x80x9d in Abstract). Vane member 37 is housed in the tubular housing so that each of an advanced-angle oil pressure chamber 51 and a delayed-dangle oil pressure chamber 52 is formed between vane portion 39 and one of two adjacent bulkhead portions 42. In accordance with engine operating condition, oil pressure is preferably supplied to and drained from each of advanced-angle oil pressure chamber 51 and delayed-angle oil pressure chamber 52. Thereby, when a high-pressure operating oil is supplied to one of advanced-angle oil pressure chamber 51 and delayed-angle oil pressure chamber 52, the tubular housing and vane member 37 make relative rotation in one rotational direction. With this, timing sprocket 25 and cam shaft 13 vary in respect of rotational phase, to thereby vary open-close timing of an intake valve 19 and an exhaust valve 20.
The valve timing control system according to Japanese Patent Unexamined Publication No. H9(1997)-324611 uses oil pressure to operate the vane member and the like which constitute a phase variation mechanism. Therefore, it is necessary to stringently control any leak of operating oil in the tubular housing in order to encourage operational response of the valve timing control system. Therefore, in order to prevent the operating oil from leaking, each component part should have high production accuracy-and-precision. However, since the tubular housing is comparatively large in dimension, the tubular housing is likely to deform during production and operation.
Sintering the tubular housing and the timing sprocket into an integrated part is under consideration recently. The tubular housing is likely to deform (into a shape of a barrel) due to temperature contraction and the like during sintering. Deformation of the tubular housing has to be prevented, Moreover, sintering the tubular housing and the timing sprocket has a difficulty in enhancing mold (compact) density higher than a predetermined level. This makes it impossible to enhance strength and mold accuracy-and-precision of a sprocket portion.
It is, therefore, an object of the present invention to provide a valve timing control system causing less operating oil leak and enhancing operational response, by securely preventing deformation of a tubular housing during production and operation of the tubular housing.
It is another object of the present invention to provide a method of producing the valve timing control system having features in the former paragraph.
According to a first aspect of the present invention, there is provided a valve timing control system. The valve timing control system comprises: a tubular housing; a cam shaft having an external periphery formed with a drive cam for operating an engine valve; a phase variation mechanism disposed in the tubular housing, and varying a rotational phase of the sprocket portion relative to the cam shaft in accordance with oil pressure supplied to the phase variation mechanism; and an oil pressure control measures for controlling the oil pressure supplied to the phase variation mechanism. The tubular housing comprises: a housing body having a density, and a sprocket portion for receiving a drive force transmitted from a crank shaft of an engine by way of a chain. The sprocket portion is disposed integrally to the tubular housing, and has a density higher than the density of the housing body. The tubular housing is so mounted to the cam shaft as to make a rotation relative to the cam shaft when so required. The cam shaft receives the drive force transmitted from the sprocket portion, to thereby rotate as a follower.
According to a second aspect of the present invention, there is provided a method of producing a valve timing control system. The method comprises the following sequential operations of: sintering a housing body of a tubular housing, and a sprocket portion of the tubular housing, so as to form an integrated sintered body; and form-rolling the sprocket portion of the sintered body so that the sprocket portion is higher in density than the housing body of the sintered body.